Electrography may be defined as an imaging process in which a latent electrostatic charge image is formed on the surface of dielectric medium and made visible by applying oppositely charged toner particles.
The inventor of the general technology, P. Selenyi, introduced the term "electrography". His classic studies are reported in P. Selenyi "On the Electrographic Recording of Fast Electrical Phenomena", Journal of Applied Physics, Vol. 9, p. 637-641, October, 1938. He describes forming an electrostatic image upon insulating plastic films supported on the surface of a rapidly moving metal surface. Charge was supplied by an image generating charge source mounted above the dielectric film. After charging, the latent charge image was developed and fixed. Selenyi formed images at the incredible recording speed of ten meters per second.
In U.S. Pat. No. 3,714,665, Mutschler et al disclose a recording system using charging styli to deposit a charge on ordinary paper moved in contact over the surface of a metal ground plane.
The use of a charge deposition source to form an electrostatic latent image on the surface of a dielectric film or paper which is moved over the surface of a conducting ground plane is described in U.S. Pat. Nos. 4,463,363 (Gundlach et al), 3,714,665 (Mutschler et al), and 4,521,791 (Day). Moving a relatively thick film or a paper web over a conducting ground plane is easily accomplished because of the high tensile strength of such materials. If, however, a relatively thin film is to be printed in registration using two or more print stations, film stretching and tensile strength considerations limit the effectiveness of this approach. In printing color images, which have large charged areas, the electrostatic tacking forces can be considerable.
Lewicki et al U.S. Pat. Nos. 5,124,730 and 5,187,501 describe a charge deposition printing system employing a plastic film temporarily affixed to the surface of an endless metal belt. In wide format printing, use of an endless metal belt becomes difficult because of problems associated with controlling belt tracking when belt width exceeds belt circumference.
The first commercial applications of electrographic printing employed direct charging of dielectric coated conducting base paper by an array of conducting styli by means of small air gap breakdown. In such processes, the rough paper surface provides an air effective gap spacing and this minimizes air breakdown voltage. Multiplexed driving of such an array of styli is possible since there is a well-defined minimum voltage for initiating electrical breakdown across small air gaps. Multiplexed addressing of writing styli is accomplished by connecting the styli in equal and electrical parallel groups, which are addressed simultaneously, thus significantly reducing the number of high voltage stylus drivers. The stylus groups are positioned adjacent a series of counter-electrodes which either contact a semi-conducting paper base or are capacitively coupled through the image receiving layer to the paper or film base. The counter-electrodes are electrically addressed in such a way as to select the stylus groups in sequence for writing. Each time a stylus group is selected for writing, the digital data for that group is applied to the stylus drivers groups in parallel but only the selected group actually puts latent image charge onto the dielectric receptor. All of the other, non-writing groups have their ability to write suppressed by applying write-suppressing voltage to the counter-electrodes. The net effect of multiplexed stylus driving is to reduce the cost of the drive electronics at the expense of writing speed since many groups must be written in sequence in order to write a single raster line having the length of the entire stylus array. In addition to speed reduction, multiplexed writing introduces a number of deleterious imaging artifacts, discussed below, which degrade the final visible image.
The paper or film base dielectric substrate which supports the dielectric image receiving layer must possess a certain minimum level of resistivity in order to prevent crosstalk between counter-electrodes yet a certain level of conductivity is necessary to enable writing at all. A narrow range of base paper resistivity between about 3 and 10 megohm per square is required of multiplexed, direct charging printers. This limited range of substrate resistivity is difficult to control resulting in imaging defects, described in more detail below. Typical base papers are described in Barr et al U.S. Pat. No. 4,868,048 and Reiche et al U.S. Pat. No. 3,995,083.
Recently introduced direct charging printers do not employ multiplexing since low cost highly integrated high voltage driver switches are now available. These printers may employ base paper having a much higher conductivity. Nevertheless, image defects remain which are associated with use of a resistive paper base. Among these are; image flares due to excess field emission, dropouts due to insufficient field emission, poor and variable dot formation due to charge spreading (blooming) caused by excess field emission, high background from toner stain caused by conductive salts offsetting from back side of the media, field-emission streaks in background areas from "off" styli, a limited variety of directly printable materials due to exacting electrical and mechanical characteristics needed for "contact" latent image creation, limited humidity range, lot-to-lot media variations, media which are difficult to manufacture consistently, printer contamination due to conductive coating which rubs off the back side of the media, printing speed which is limited by base sheet resistivity, and poor sheet aesthetics due to the "chemical" coating on the back side. When multiplexed writing is employed, additional image defects occur which are commonly referred to as "multiplexing striations", speeds are further reduced, and the other imaging defects inherent in all contact writing are considerably exacerbated
Early applications of small-gap-breakdown charging of dielectric paper were in the plotter field. Developments over the last several years have seen resolution increased to 16 points per millimeter, printing widths growing to 1.8 meters, and the availability of color.
Electrostatic printing technology was originally developed for text printing and CAD applications. Its advantages of speed, image durability, and good color gamut at moderate cost led to its early dominance in wide-format graphics' applications. The problems listed above, however, are currently causing its market share, in terms of both printer equipment and square footage of output, to erode in favor of simpler, but slower, methods such as ink jet. The total amount of printed output produced using electrostatic printers will continue to increase for the next several years in spite of these problems but, clearly, what is needed is a technology with the advantages of electrostatics but without its drawbacks.
Another method of forming electrostatic latent images employs a gated charge source spaced a tenth of a millimeter or more from the dielectric receptor surface. Here, a charge is formed using a low energy spark, corona wire, or silent electric discharge and the flow of these charges to the recording surface is controlled by fields other than those directly responsible for charge generation.
The silent electric discharge method of latent image charge generation has been commercially successful. This technology has been referred to as; ion printing, charge deposition printing, ion projection printing, and electron beam imaging. Fotland et al U.S. Pat. Nos. 4,155,093 and Carrish 4,160,257 disclose this charge image generation method.
Charge deposition printing provides for very high current density imaging that, in turn, allows one to print electrostatic images at extremely high speeds. Commercial printers presently operate as fast as 2.3 meters per second speed and even higher speeds have been attained in the laboratory. An image element, or dot, may be written in periods as short as 100 nanoseconds. In this case, the time constant of the dielectric receptor media must be less than about 10 nanoseconds. Since the relative dielectric constant of paper is approximately 2, the paper resistivity must be less than 5 megohm-cm. in order to prevent transient voltage drops across the paper base. Such voltage drops have the effect of reducing the extraction electric field of the charge deposition printhead
Presently employed dielectric media, and particularly dielectric coated paper, exhibit the many problems listed above when employed with either high speed charge deposition imaging systems or non-multiplexed direct charging apparatus.